release-21.2: opt: match ScalarGroupBy in EliminateIndexJoinOrProjectInsideGroupBy

[blathers-crl]

Backport 1/1 commits from #70120 on behalf of @mgartner.

/cc @cockroachdb/release

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This commit extends EliminateIndexJoinOrProjectInsideGroupBy so that
ScalarGroupBy expressions are also matched. This allows the rule to
eliminate unnecessary index joins in more cases.

The primary motivation for this change was to make partial index
validation queries more efficient. These queries always have
ScalarGroupBy expressions because they are in the form:

SELECT count(1) FROM table@partial_index WHERE predicate

Prior to this change, an index join was planned for these queries which
would operate on every row in the partial index. This could be extremely
expensive for large partial indexes.

Fixes #70116

Release note (performance improvement): A limitation has been fixed that
made creating partial indexes inefficient.

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Release justification: This fixes a major performance issue with partial indexes.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[rytaft]

Is this fixing a release blocker? If not, it should probably wait until 21.2.1

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @RaduBerinde)

[mgartner]

It's not a blocker so I'll hold off on merging.